An Electron-Ray Titrimeter J
EARL J. SERFASS, Lehigh
University,
Bethlehem, Penna.
The unrectified circuit described is particularly convenient for use with polarized electrodes, whereas the rectified circuit may be used for titration systems requiring the use of the calomel cell. Both units may be assembled from ordinary radio replacement parts for the cost of less than a small portable radio receiver.
Two simple thermionic voltmeters which operate from the alternating current or direct current lighting circuit are described. These voltmeters are particularly suited for potentiometric titrations, since an inexpensive electron-ray tuning indicator tube is used as an indicator instead of the usual microammeter.
which employ the principles of mu-balance in combination with a Kheatstone bridge circuit in order to obtain stability, are unusually simple in construction and operation. The recent battery-operated voltmeter contributed by Garman and Droz (3) is not only exceedingly simple in construction but is designed for very low current consumption, which eliminates the necessity for frequent battery replacement. Despite the remarkable advances made in the design of both line- and battery-operated equipment, all the vacuum tube voltmeters used for chemical purposes retained one practical disadvantage-namely, the use of a sensitive galvanometer or microammeter. The elimination of the somewhat fragile and expensive indicator was pioneered by Smith ( 7 ) ,who presented a circuit in which a cathode-ray tuning indicator (Magic Eye) was substituted for the microammeter. I n this rather useful circuit the high-voltage direct current necessary for the operation of the amplifier and eye tubes is obtained by the conventional transformer, rectifier, and filter circuit. The inclusion of a rather elaborate mechanism for the control of the eye position causes the circuit to be so comdex as to discourage the average chemist with a minimum knowledge of vacuum tubes from attemptR-1 ing to construct the titrimeter. By using a circuit somewhat new in principle not only is construction simplified but the cost of parts is also considerably reduced. This present paper presents an alternating current-direct current transformerless, line-operated titrimeter which is sufficiently stable and sensitive for purposes of titration without the use of the principles of mubalance. I n addition to the simple unrectified circuit, a rectified model which exhibits a low grid current drain is also presented.
LECTROS tube voltmeters designed specifically for use as contiiious reading devices for electrometric measurements have almost universally replaced, for general chemical purposes, the conventional potentiometric system. Since the first battery-operated thermionic titrimeters were proposed by Goode ( 4 5 ) many similar and improved types have appeared. Recently the trend has been toward the design of alternating currentdirect current line-operated equipment. These numerous, published circuits are not due to the general unsatisfactory operation of vacuum tube voltmeters but rather are the result of improvements in design directed toward increased sensitivity and stability. Unfortunately improvements in sensitivity and stability and other advancements in electrical engineering have contributed to the complexity of many circuits. The 8-tube circuit of Rescorla, Carnahan, and Fenske (6),although stable to line voltage variation and sufficiently sensitive, is so expensive and complex as to discourage its use b y the average chemist. On the other hand, the circuits developed by Garman and Droz (2) and Willard and Hager (8), both of
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Unrectified Circuit The schematic diagram of the unrectified circuit is shown in Figure 1.
SOCKET
CONNECTIONS
REPRESEXTATION OF SIMPLE TITRIMETER FIGURE1. SCHEMATIC E-1. 500,000-ohm fixed resistor, 0.5 watt
R-2. 350-ohm line cord resistor (Clarostat PC* 350) R-3. 25,000-ohm volume control (2-watt Sfidpet) R-4. 25,000-ohm fixed resistor
E-5. 1000-ohm fixed resistor, 0 . 5 watt R-6. 5000-ohm volume control @-watt Midget) R-7. 25,000-ohm fixed resistor (may be omitted) 9-1. Switch cover plate attached to E-3
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.i simple triode, all-metal 6F5 radio vacuum tube is used as a voltage amplifier to step up the voltage betwen the electrodes of the external cell. This amplified voltage differenceis used to con-
SEPTEMBER 15, 1940
337
ANALYTICAL EDITION
s-1
twin diode detector tube, 6H6, is used in conjunction with the filter condenser, (2-1. Provibion has been made in this circuit for its use with externally polarized electrodes. A small polarizing current is taken from the cathode load of the GF5 by means of resistors R-8 and R-9 which may be controlled by the switch, 8-2
Operation as a Titrimeter The majority of chemi4+ are familiar with the apRl P-2 pearance of the electron-ray tuning indicator (Magic Eye) of the modern radio Pl K h e n 100 volts are applied H H to the plate and target of the tube, the target will 1 I0A.C. OR D.C. exhibit a green fluorescence s K-2 over its entire surface, providing the grid is maintained FIGURE2 . SCHEMATIC REPRESESTATIOS O F RECTIFIED TITRIMETEE CIKCLIT a t a potential 3.3 volt. 8 - 6 . 3000-ohm volume control ( 2 - n a t t IIidget) C-1. 4-mid. electrolytic condenser, 400 volts 11-8. 1500-ohm fixed resistor, 0.5 w a t t R-1. 500,000-ohm fixed resistor, 0.5 w a t t more negative than the R-9. 100,000-ohm fixed resistor, 0.5 watt R - 2 . 330-ohm line cord resistor (Clarostat PC* 330) cathode. If the grid iS-1.Switch cover plate attached to R-3 R-3. 25,000-ohm volume control (2-watt Midaetl R - 4 . 25,000-ohm fixed resistor, 0.5 w a t t S - 2 . Single-pole, single-throw toggle switrh maintained a t the same poR - 5 . 1000-ohm fixed rpsiptnrq 0.5 watt tential as the cathode (zero bias) a shadow of amroxitrol the grid of the electron-ray tuning indicator, 6E5. Since niately 100" will be thrown 011 the fluorescent target. For all both the 6F5 and 6E5 were found to operate satisfactorily with practical purposes the shadow angle mill be found to be a lineal alternating current on their plates, it was found unnecessary in function of the grid bias applied from 0 to -3.3 volts. most cases to transform or rectify the 110-volt alternating current line supply. Furthermore, the filament current for both Let us assume that a titration cellis connected to the titrimrtubes is supplied from the line voltage by means of a standard ter with a n initial potential of 0.0 volt. The potential line cord resistor of t,he type used in transformerless radio sets. across R-4, which is directly applied to the grid of the 6E3, The variable cathode load resistor, R-3, is used to govern t'he may be offset by adjustment of R-3 so that the potential grid bias of the 6E5 indicator and as such this control is used to adjust the position of the shadow on the target of the eye at the difference between the cathode and grid of the 6E5 is zero. start of a titration. The variable resistor, R-6, in series with regardless of the initial setting of the sensitivity control, R-6 the cathode of the 6F5 governs the grid bias of the amplifier and Under these conditions the eye will remain open-i. e., full as such this control is used to adjust the sensitivitxof the titrimeshadow. If, during the course of the titration, a small negater as a whole. .4s the resistance of R-6 is increased in value, the potential drop across R-5 and R-6 is increased, which results tive potential is applied to the grid of the 6F5 by means of the in the application of a higher negative grid bias on the 6F5, procell electrodes, the greatly decreased current flowing through vided the externally applied potential is not altered. Since the the cathode circuit of the 6F5 m-ill cause the grid of the 6E5 6F5 is a sharp cutoff tube, an increase in negative grid bias above to become negative with respect to its cathode. The net rethe normal operating bias of -2 volts will cause a marked decrease not only in plate current but also in the gain or voltage sult of the applied potential will be a closing of the eye-i.
